This invention relates to the field of radiotherapy, and more particularly to the field of intra-arterial stents that are used for implanting at or adjacent the side of treatment of cancer cells or other patholitical conditions in a human body.
The intra-arterial stents have been widely used to prevent restenosis following angioplasty, wherein a coronary artery is dilated with a balloon. In time, the artery widened by an implanted balloon tends to close due to elastic recoil of the tissue. In the past, one of the solutions was to use a stent with radiation material embodied into the body of the implant in order to maintain patency of the artery following angioplasty. However, implantation of the stent often leads to intimal hyperplasia in about six months and causes closure of the artery again.
One example of a stent which utilizes a radioisotope imbedded into an arterial stent has been disclosed in U.S. Pat. No. 5,059,166 issued on Oct. 22, 1991 to Fischell et al. In that patent, the stent is implanted at a place in the artery wherein it irradiates a tissue in close proximity to the implantation site of the stent and reduces the tissue growth following a balloon angioplasty. The stent according to '166 patent has a cross section of two turns of a coil spring fabricated from a pure metal or alloy, which has been irradiated to become radioactive. The radioisotope used in '166 patent may be an alpha-beta or gamma emitter. The half life is between 10 hours and 100 days. The most preferable embodiment would have a half life of 16 days.
One of the disadvantages of this type of stent is that it presents a considerable storage problem. The half life of the isotope is very limited, and a surgeon or radiation oncologist has to keep a continuously fresh supply of stents ready for his use during an implantation surgery. Once the isotope has lost its effectiveness, the stent has to be discarded and new stents have to be purchased. Therefore, the physician is required to predict how many stents he would require during a particular period of time and order only the required amount, so as to not waste the valuable devices.
Another example of a radioactive stent is disclosed in U.S. Pat. No. 5,213,561 issued on May 25, 1993. In that patent, the radioactive source is mounted at the distal end of a guide wire, or in a balloon catheter. A balloon expansible stent as is inserted through the vascular structure to the site of radiation treatment. The flexible member is inserted longitudinally through the structure and a radioactive source mounted at the distal end of the flexible member becomes placed at the end of the tube, inside the balloon. An outer sleeve of the guide wire slides over the inner wire for a distance sufficient to cover and uncover radioactive material, so that the shield formed by the outer sleeve can be moved away from the radioactive material and expose the angioplasty site to radiation. After the exposure, the outer sleeve is shifted again to cover the radioactive section. The device is designed to selectively prevent exposure of the walls of the vascular structure when the guide wires are inserted or being removed.
Still another example of a radioactive stent is disclosed in U.S. Pat. No. 5,572,984 issued on Mar. 3, 1998 to Fischell et al. There, a radioactive coating on an embedded radioactive isotope performs both antithrombogenic and radioactive function. The preferred embodiment of '984 patent discloses a phosphorylcholine coated stent where some of the phosphate groups contain phosphorus 32. Another example of a stent prepared with a radioactive coating is disclosed in U.S. Pat. No. 5,176,617 issued on Jan. 5, 1993 to Fischell et al. In that patent, the stent is a tubular, thin-walled structure that extends radially outward against the wall of the vessel, with a part of the stent being formed from a radioisotope material.
In all these patents, the radioisotope is integral to a stent and is designed to irradiate tissue in close proximity to the implantation site of the stent. Some of the devices are designed to reduce malignant cell growth in a blood vessel, for example a bile duct, while others are designed to maintain vessel patency without injuring the surrounding tissue. All of the above patents suffer from the same disadvantage; they have a limited shelf life, and a supply of them must be continuously replaced in order to provide more benefits to the patient.
The present invention contemplates elimination of drawbacks associated with the prior art and provision of a radiotherapy stent that has a more or less unlimited shelf life and can be loaded with radioactive material immediately prior to positioning in a human body.